It has been known heretofore that electrical parameters governing gel electrophoresis, i.e., electrophoresis on a flat surface of a gel, in a buffer solution, include the magnitude of the electric field, the direction of the electric field, and the evolution of the magnitude and direction of the electric field as functions of time, as well as the type and temperature of the buffer solution, the material and configuration of the electrodes, and other factors. Both homogeneous and nonhomogeneous electric fields have been used in gel electrophoresis. An electric field is regarded as homogeneous if it is uniform, in magnitude and direction, across the flat surface of the gel at any moment in time.
It also has been known heretofore that pulsed modulation of the magnitude of the electric field, in its direction, or both, enhances the utility of gel electrophoresis, particularly in the separation of proteins, nucleic acids, and other such macromolecules. Pulsed modulation refers to modulation at timed intervals, between which the electric field may be substantially uniform across the flat surface of the gel.
In recent years, apparatus for gel electrophoresis have advanced from apparatus employing two parallel driving electrodes, or two parallel arrays of driving electrodes, which apply electric fields tending to cause molecules to migrate along straight lines between such electrodes, to apparatus employing polygonal arrays of driving electrodes, which apply electric fields that can be directionally modulated so as to tend to cause molecules to migrate along non-straight paths, e.g., paths having corners or zig-zag paths.
In one heretofore known type of apparatus for gel electrophoresis, as mentioned above, a square array of driving electrodes is provided, which enables electric fields to be alternatively applied in transverse directions so as to tend to cause samples being electrophoresed to migrate in paths that change direction at 90.degree. angles. See, e.g., Cantor et al. U.S. Pat. No. 4,473,452, which discloses that such fields can be substantially uniform, if applied by and between paired electrodes in like numbers on opposite sides of the square array, or substantially fan-shaped, if applied by and between one electrode on one end of a given side of the square array and plural electrodes on the opposite side of the square array. This patent also mentions that each electrode can be selectively maintained at any positive or negative electrical potential within a selected range.
In another heretofore known type of apparatus for gel electrophoresis, as mentioned above, a hexagonal array of driving electrodes is provided, which enables homogeneous electrical fields to be alternatingly applied at timed intervals so as to cause molecules being electrophoresed to migrate in zig-zag paths. See, e.g., Biotechnology, December 1986, page 1054, which refers to electrophoresis in such apparatus as contour-clamped homogeneous electric field (CHEF) electrophoresis, and which compares CHEF electrophoresis to other heretofore known techniques including pulsed-field gel electrophoresis (PFGE) and field-inversion electrophoresis. See, also, Chu et al., "Separation of Large DNA Molecules by Contour-Clamped Homogenous Electric Fields," Science, Dec. 19, 1986, Vol. 234, pp. 1582-5. An apparatus employing parallel driving electrodes, for field-inversion electrophoresis, is disclosed in Carle et al. U.S. Pat. No. 4,737,251.
Although some of the heretofore known apparatus and techniques discussed in the preceding paragraphs have been valuable contributions to the art of electrophoresis, there has remained a need, to which this invention is addressed, for greater flexibility in directional modulation of an electric field in an apparatus for electrophoresis.